[Discover] Improve sample queries for SQL and PPL

[joshuali925]

Is your feature request related to a problem? Please describe.

#8752 (comment)

• field names should be quoted
• some examples are a bit duplicate and overlapping
• need to verify if the queries are supported by all dataset types
• some description are inaccurate, for example in [Discover] add sample queries for PPL #8752 (comment)

Describe the solution you'd like

reword the sample queries

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Additional context

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